Photoactivatable glycopeptide reagents for site-specific labeling of lectins

Photoactivatable, iodinated glycopeptides bearing oligosaccharides of defined structure have been synthesized for use as lectin binding site-specific agents. Two such glycopeptides have been examined utilizing concanavalin A, RCAI (Ricinus communis agglutinin), RCAII (R. communis toxin), and the Gal/GalNAc-specific lectin from human and rat hepatocytes. Covalent incorporation upon photoactivation only occurs with a glycopeptide which is specifically bound by the lectin and is inhibited only by appropriate haptenic monosaccharides. The efficiency of covalent coupling is on the order of 2%. Half-maximal covalent incorporation occurs at a concentration of photoactivatable glycopeptide which corresponds to the previously determined association constant for lectin binding in each case. Covalent incorporation of glycopeptide is accompanied by a decrease in the relative mobility of the lectin upon sodium dodecyl sulfate polyacrylamide gel electrophoresis equivalent to that expected for addition of an oligosaccharide moiety. The high degree of specificity and the relatively high affinity of these photoactivatable glycopeptides makes them promising agents for the examination of lectins such as the Gal/GalNAc-specific receptor present on mammalian hepatocytes.